The involvement of tumor growth factor-beta (TGF-beta) in embryonal development of the mouse was investigated in embryos of 10 to 18 days of gestation, using antibodies raised against synthetic peptides of the TGF-s monomer to localize the growth factor. TGF- beta was found in a variety of tissues of ectodermal and mesenchymal origin, predominantly around day 15 when organogenesis is most intense. The wide distribution of TGF-beta-indicates its involvement as a regulator in major events of cytodifferentiation. As TGF-beta is known to be a modulator of the extracellular matrix, the relationship of the growth Factor to the synthesis of matrix components (fibronectin, collagen I, laminin) was evaluated in the developing mouse embryo.